1. Field of the Invention
This invention relates generally to color printers, and particularly relates to color image registration systems for sequential xerographic image transfer.
2. Discussion of the Related Art
The basic xerographic process includes exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged to record an electrostatic latent image corresponding to the original document. In black and white printing, a development system moves a developer mix of carrier granules and black toner particles into contact with the photoconductor surface. The toner particles are attracted electrostatically from the carrier granules to the latent image, forming a toner powder image thereon. The toner powder image is then transferred to a sheet of paper or other support material. This sheet of paper advances to a fuser which permanently affixes the toner powder image to the paper.
Multi-color electrophotographic printing is substantially the same as the process for black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding respectively to different colors are recorded. Each single color latent image is developed with the appropriately colored toner. The single color toner images are transferred to the copy sheet in superimposed registration with one another, creating a multi-layered toner image on the copy sheet. The multilayered toner image is then permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid material or a powder material.
In order to successively register toner images of different colors in a superimposed manner, a paper transport system was included in the basic xerographic process. The paper transport system moved the copy sheet in a recirculating path relative to the photoconductive member so that successive toner images were transferred in superimposed registration with one another.
One type of paper transport system included an electrically biased drum rotating in synchronism with the photoconductive member. The copy sheet was secured to the drum either by gripper bars or by vacuum tacking and rotated with the drum. Another, more recent type of paper transport system moved a vacuum belt in a recirculating path around a stationary vacuum chamber. The moving belt carried the copy sheet to a transfer zone where a toner image was transferred to the copy sheet by the photoconductive member. The copy sheet was then carried by the moving belt around the stationary vacuum chamber for successive toner image transfer onto the copy sheet. This type of paper transport system is disclosed in commonly assigned U.S. Pat. No. 5,081,506, which issued to the present inventor on Jan. 14, 1992, and is incorporated herein by reference.
Because the successive toner images of different colors are superimposed on each other to form a copy of a single colored image, it is necessary to control accurately the registration of images on the paper to ensure that the successive toner images are directly superimposed on each other without overlap. If done inaccurately, a blurred and shaded single or composite image will be formed.
Generally, color registration systems register the paper to the image formed on the photoconductive member. This is done by controlling the movement of the paper about the transport system so that the paper passes through the transfer zone simultaneously with the image formed on the photoconductive member. A paper to image color registration system can generally achieve an image resolution of approximately an 0.005 inch circle.
The paper generally passes through the transfer zone simultaneously with the image by rotating the transport system synchronously with and at the same speed as the photoconductive member. However, if the photoconductive member is compressed or elongated during rotation, changes in the surface velocity will occur, resulting in the photoconductive member and image rotating out of phase with the transport system.
If the paper moving around the transport system becomes out of synchronism with the image being formed on the photoconductive member, the paper is either accelerated or decelerated to the transfer zone by controlling the velocity of the transport system. This generally requires the use of servomotors to precisely control the acceleration and deceleration of the transport system. As a result, the transport system, in conjunction with the photoconductive member, becomes bulky and very expensive. This is undesirable in light of the need for smaller, less expensive color printers.